In a turbo-machine, such as a gas or steam turbine, rows of blades project radially outwardly from the circumferences of respective rotor disks that are, in turn, attached along a length of an axially aligned shaft. Each blade extends radially from a rotor disk and is affixed at its root to the disk by a mechanical connection. An airfoil portion of each blade reacts to the forces of a working fluid flowing axially through the machine to produce rotation of the rotor, thereby extracting mechanical shaft power from the working fluid. The blades experience steady state centrifugal forces, bending moments and alternating forces during operation. In addition, blade vibration from alternating forces will generate significant stresses on the attachment structure.
Blades are attached to the rotor disk with one of two styles of mechanical connections: an axial attachment or a radial attachment. FIG. 1 is a perspective view of one embodiment of an axial (side entry) blade attachment mechanism for a turbo-machine. A turbine rotor disk 2 is formed to have a plurality of equally spaced axially oriented grooves 4 disposed around its circumference. Each groove 4 is individually milled or broached to a predetermined shape, such as the fir tree design of FIG. 1. Blades 6a, 6b, 6c are disposed about the circumference of the rotor disk 2, each blade 6 having a root portion 8 formed for sliding side entry into a respective groove 4 of the disk 2. The platform portions 10 of adjacent blades define one side of a flow path for the working fluid as it passes through the airfoil portions 12 of the blades. In most embodiment, shrouds (not illustrated) are disposed along the outer circumference of the airfoils to create a mechanical connection between the blades. There is generally no contact between platforms of adjacent blades 6a, 6b, and 6c . Examples of axial blade attachments may be found in U.S. Pat. Nos. 3,501,249 and 5,176,500, both incorporated by reference herein.
FIG. 2 is a perspective view of one embodiment of a prior art radial entry blade attachment mechanism for a turbo-machine. A rotor disk 14 has a single continuous groove 16 formed around its circumference. One will appreciate that the manufacturing cost for forming such a continuous groove 16 is significantly less than the manufacturing cost for forming the individual axial grooves 4 described in FIG. 1. The radial groove 16 of FIG. 2 has a female fir tree shape, although other shapes, including a male fir tree shape and a T-shank shape, are known. Each blade 18a, 18b has a mating male root portion 20 that is engaged within the rotor disk groove 16.
FIG. 3 is a perspective view of a second embodiment of a prior art radial entry blade attachment mechanism. A rotor disk 24 is formed to have a continuous T-shank shape 26 around its circumference in lieu of the groove 16 of FIG. 2. The root portions 28 of each of the blades 30a, 30b have a mating T-shank shaped groove 32 formed therein. The blades 30a, 30b are individually installed onto the rotor disk 24 at an entering slot location. The entering slot location is not illustrated in FIG. 3, however, an exemplary entering slot location 34 for a fir tree design radial entry rotor disk 25 is shown in FIG. 4. One entering slot location 34 or two diametrically opposed entering slot locations may be used. The lugs of the T-shank shape 26 (or fir tree shape 26 as appropriate) are missing at the entering slot location so as to allow the blades to be moved into position in a radial direction. The blades are then free to be slid circumferentially around the perimeter of the rotor disk 24 from the entering slot location to their final installed position as illustrated in FIG. 3. The blades 30a, 30b make contact with each other at the root portion 28 when a full complement of blades 30 is installed.
Once a full complement of blades is installed onto a radial entry disk, a closing blade 36, as illustrated in FIG. 5 for a fir tree design, must be installed into the entering slot location 34. One or more pins (not shown) are installed through respective mating holes 38, 40 formed in the rotor disk 25 at the entering slot location 34 and in the closing blade 36 to provide a radial attachment mechanism. The pins function to resist the centrifugal forces generated during operation of the machine, since the lugs of the fir tree shape are missing at the closing piece location 34. Examples of radial blade attachments may be found in U.S. Pat. Nos. 4,915,587 and 5,176,500, both incorporated by reference herein.
While radial entry blade attachment is often a more economical choice than axial blade attachment, it is known that the stresses imposed upon the pins of the closing blade attachment are higher than those experienced in the lugs of the adjoining blades. For some large blade configurations or high speed rotors, the stresses are so high that the closing blade 36 must be replaced with a closing piece 42, such as the one illustrated in FIG. 6. The closing piece 42 has the same root/platform portion 44 as the closing blade 36 but it lacks an airfoil portion and thus generates relatively little centrifugal force during operation of the turbine. In order to maintain the turbine rotor in balance when a closing piece 42 is installed in the entering slot location 34, a filling piece 46 as illustrated in FIG. 7 may be installed in lieu of a blade 30 at the location diametrically opposed to the entering slot location 34. While this approach solves the problem of high stress levels at the closing location, it results in a decrease in turbine efficiency due to the two missing airfoil portions in each row of blades. Furthermore, the perturbations of the working fluid flow created by the missing blades cause an increase in the alternating stress levels on the blades and blade attachments. This effect may be exacerbated because an outer shroud (not shown) connected to each blade at their respective radially outermost ends 48 can not span an entire 360° arc; but rather, because of the missing airfoil portions, may be formed into two sections each spanning somewhat less than a 180° arc. Accordingly, bending moments and the alternating stress levels in all of the blades are adversely affected by the absence of two airfoil portions within the row of blades.
U.S. Pat. No. 4,094,615, incorporated by reference herein, describes a blade attachment arrangement for the ceramic blades of a high temperature gas turbine engine. Ceramic material does not exhibit a high tensile strength, and a standard blade attachment arrangement is not acceptable for this application. Accordingly, each blade is attached to the rotor disk via an individual metallic attachment member. The turbine disk in this arrangement is fabricated to have a plurality of axial grooves along its circumference, as in the typical axial blade attachment arrangement described above. The metallic attachment members each have a root portion for engaging a mating groove of the rotor. The attachment members also each have an outer peripheral groove for receiving a root of a corresponding ceramic blade. Opposed slots are formed in the attachment members and the blade platforms for receiving metal plates that transfer torque from the blades to the corresponding attachment piece, thereby reducing stress levels in the ceramic blade roots. The attachment piece and the metal plates combine to support the blade during operation. In addition, a second series of opposed plates is required to protect the attachment from the high temperatures. This blade attachment arrangement is complicated and expensive and would not be desirable for a standard metallic turbine blade application.